This application claims priority to an application entitled xe2x80x9cERBIUM-DOPED OPTICAL FIBER HAVING GRATINGS FORMED THEREIN AND FABRICATING METHOD THEREOFxe2x80x9d filed in the Korean Industrial Property Office on Mar. 11, 1999 and assigned Serial No. 99-8079, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an erbium-doped fiber amplifier (EDFA) for amplifying an optical signal, and in particular, to an erbium-doped fiber amplifier having a gain flattening function and a fabricating method thereof.
2. Description of the Related Art
An erbium-doped fiber amplifier (EDFA) is used to amplify an optical signal to compensate for the attenuation caused by long distance transmission of a large amount of data on a single strand of optical fiber. The erbium-doped fiber amplifier is advantageous in terms of amplification efficiency and cost because the erbium-doped fiber amplifier directly amplifies an optical signal without the need of the steps of opto-electric conversion, amplification, and then electro-optic conversion for data transmission. The erbium-doped fiber amplifier, however, has a different amplification gain at each wavelength and there is a need to flatten the gain.
For gain flattening in a conventional erbium-doped fiber amplifier, long-period fiber gratings (LPFGs) are formed in an optically sensitive fiber and the optical fiber is spliced with an erbium-doped fiber (EDF). The splicing of the optical fiber with long-period fiber gratings and the erbium-doped optical fiber with a small core radius causes loss.
Examples of erbium-doped fiber amplifiers of the contemporary art are seen in the following U.S. Patents. U.S. Pat. No. 5,337,382, to Mizrahi, entitled Article Comprising An Optical Waveguide With In-Line Refractive Index Grating, describes broadband in-line gratings in optical waveguides. The patent describes an amplifier including an Er-doped optical fiber which is illustrated with a grating toward one end of the fiber. However, the described gratings are short period, broadband gratings with high reflectivity. The grating may be a blazed grating or a tilted grating.
U.S. Pat. No. 5,892,615, to Grubb et al., entitled Output Power Enhancement In Optical Fiber Lasers, describes an optical fiber as the active amplifying medium in a fiber laser. The core of the fiber may be doped with erbium, and the core may have a long period grating formed therein. The grating is for producing a large distributed loss for suppressing the Raman frequency in the fiber. The grating has a relatively high insertion loss at the Raman frequency while simultaneously providing a relatively low insertion loss at the lasing frequency. The grating is formed near the center of the fiber and is illustrated as spaced between loops of the fiber.
U.S. Pat. No. 5,920,582, to Byron, entitled Cladding Mode Pumped Amplifier, describes an optical waveguide having an erbium-doped core, an inner cladding and an outer cladding. The inner cladding may include germanium as a dopant. A long period index grating is formed in the inner cladding, being written by an ultra violet source.
U.S. Pat. No. 5,966,481, to Jolley et al., entitled Optically Pumped Optical Waveguide Amplifier, describes an optically amplifying waveguide with a central portion doped with erbium ions. The doped central portion is provided with a long monotonically chirped period grating created by micro stretching.
It is, therefore, an object of the present invention to provide an improved erbium-doped fiber amplifier.
It is a further object of the present invention to provide an improved erbium-doped fiber for use in an erbium-doped fiber amplifier.
A still further object of the invention is to provide an erbium-doped fiber amplifier with reduced loss due to splicing.
A yet further object of the present invention is to provide an erbium-doped fiber amplifier with a gain flattening function.
A still further object of the invention is to provide an erbium-doped fiber for use in fabrication of a gain flattening erbium-doped fiber amplifier without using an extra filter.
Another object of the present invention is to provide an erbium-doped fiber amplifier having fewer parts.
These and other objects can be achieved by providing an erbium-doped optical fiber (EDF) and a fabricating method thereof. The erbium-doped optical fiber has a core formed by substantially doping silica with erbium, has gratings formed in the core at a predetermined period for propagating light therethrough, and has a cladding surrounding the core and having a lower refractive index than the core.
According to another aspect of the present invention, there is provided an erbium-doped fiber amplifier (EDFA) fabricating method. For fabrication of the erbium-doped fiber amplifier, an optical fiber preform is formed to include a core formed by substantially doping silica with erbium and a cladding substantially formed of silica. Then, an optical fiber is drawn by applying a predetermined tensile force to the optical preform and gratings are written in the drawn optical fiber by relieving residual stresses of the optical fiber through annealing of the optical fiber at a predetermined spacing.
According to a further aspect of the present invention, there is provide an erbium-doped fiber amplifier for amplifying an input optical signal. In the erbium-doped fiber amplifier, a pump light source generates a pump light at a predetermined wavelength, an optical coupler receives the pump light from the pump light source and couples the pump light with the input optical signal, and an optical fiber has a core which is formed by substantially doping silica with erbium, and has gratings formed therein at a predetermined period for amplifying the pumped optical signal.